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    SubjectRe: [00/41] Large Blocksize Support V7 (adds memmap support)
    On Tue, Sep 11, 2007 at 04:00:17PM +1000, Nick Piggin wrote:
    > > > OTOH, I'm not sure how much buy-in there was from the filesystems guys.
    > > > Particularly Christoph H and XFS (which is strange because they already
    > > > do vmapping in places).
    > >
    > > I think they use vmapping because they have to, not because they want
    > > to. They might be a lot happier with fsblock if it used contiguous pages
    > > for large blocks whenever possible - I don't know for sure. The metadata
    > > accessors they might be unhappy with because it's inconvenient but as
    > > Christoph Hellwig pointed out at VM/FS, the filesystems who really care
    > > will convert.
    >
    > Sure, they would rather not to. But there are also a lot of ways you can
    > improve vmap more than what XFS does (or probably what darwin does)
    > (more persistence for cached objects, and batched invalidates for example).

    XFS already has persistence across the object life time (which can be many
    tens of seconds for a frequently used buffer) and it also does batched
    unmapping of objects as well.

    > There are also a lot of trivial things you can do to make a lot of those
    > accesses not require vmaps (and less trivial things, but even such things
    > as binary searches over multiple pages should be quite possible with a bit
    > of logic).

    Yes, we already do the many of these things (via xfs_buf_offset()), but
    that is not good enough for something like a memcpy that spans multiple
    pages in a large block (think btree block compaction, splits and recombines).

    IOWs, we already play these vmap harm-minimisation games in the places
    where we can, but still the overhead is high and something we'd prefer
    to be able to avoid.

    Cheers,

    Dave.
    --
    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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